Neutron diffraction study of the β' and γ phases of LiFeO2

The β, β', γ and α phases of LiFeO2, synthesized as powders, were annealed at different temperatures and characterized by X-ray measurements. The β' and γ modifications were also studied by time-of-flight neutron diffraction (ISIS Facility, UK). The structure of the β' phase was refined in the monoclinic C2/c space group (a=8.566(1), b=11.574(2), c=5.1970(5) A, β=146.064(5)o) to wRp=0.071-0.080 (data from four counter banks). Fe and Li atoms are ordered over two of the four independent sites, and partially disordered over the other two. The ordered Li has a distorted tetrahedral coordination. The γ structure was refined at RT (a=4.047(1), c=8.746(2) A) and at 570 deg. C (a=4.082(3), c=8.822(6) A) in the I41/amd symmetry, showing full order with Li in octahedral coordination at RT, and in a split-atom configuration at high temperature. On annealing, the β' polymorph was found to transform to γ at 550 deg. C, thus suggesting that it is a metastable phase. Electrostatics is discussed as the driving force for the α→β'→γ ordering process of LiFeO2. - Graphical abstract: Crystal structure of β'-LiFeO2 (monoclinic C2/c). Lithium and iron atoms are both ordered (blue and yellow balls) and partially disordered (green balls) over four independent sites. The β' phase transforms to fully ordered γ (tetragonal I41/amd) at 550 deg. C.